In general, an OFDMA system uses an adaptive modulation and coding method, which selects a different modulation and coding method depending on a change of channel states per sub-channel while performing a speed packet communication.
FIG. 1 shows an example of a packet in which an adaptive modulation and coding method is applied to in a general OFDMA system.
As referred to in FIG. 1, at least one of preamble symbol are is located to the head of each packet, and are used for acquirement of initial synchronization, calculation of frequency offset, and recognition of Cell ID, etc.
Data symbols of the packet follow the preamble, and several continuous data symbols gather and form one slot. The reason for forming the slot is that at least 3 symbols are needed to assign a data block given to a channel codec to one sub-channel, since the length of one symbol is not enough for the assignment.
Next, a frame prefix is located in the front of the data slot, this information is being constructed by the QPSK (Quadrature Phase Shift Keying) method, and it includes a location information and size information of a MAP information. Following the frame prefix, a DL MAP and a UL MAP are located, and the DL MAP describes a modulation method and a channel coding method applied to each sub-channel in a downlink, and the UL MAP describes a modulation method and a channel coding method applied to each sub-channel in an uplink. Data bursts following the DL MAP and the UL MAP are modulated by various methods predetermined in the MAP.
This frame construction is adopted by IEEE 802.16a.
FIG. 2 shows a construction of a modulator for modulating a packet to which an adaptive modulation and coding method is applied to in a general OFDMA system.
As referred to in FIG. 2, the received OFDMA packet is transformed in a Fast Fourier Transform (FFT) device by an FFT method, and a channel for the packet is estimated and is equalized in an equalizer 23 through a re-ordering buffer 22. The channel is QAM demapped in a QAM (Quadrature Amplitude Modulation) demapper 25 and is channel decoded in a channel decoder 27 through a slot buffer 26, and is finally demodulated.
Meanwhile, there are several buffers 24 the before QAM demapper 25. Since modulation methods for each sub-channel are described in the MAP area, it is necessary to understand the contents for the QAM demapping process. Thus the data symbols are needed to be delayed in the buffers during when the channel decoder 27 analyzes the frame prefix and the MAP information.
FIG. 3 shows action timing for modulating a packet in the modulator shown in FIG. 2. FIG. 3 indicates an order and the time required for modulating the packet frame shown in FIG. 1 by the modulator of FIG. 2.
As referred to in FIG. 3, the delay by the FFT 21 and the delay by the re-ordering buffer 22 corresponds to each symbols, and in the case all the FFT for the first slot are is finished, the information of the frame prefix are is analyzed by the channel decoder 27 and the location and size of the MAP information can be found. First, It is needed to perform the QAM demapping process and the channel decoding to the sub-channels on which the MAP information are loaded on, and determine the modulation method and the channel coding method for each sub-channels, in order to perform the QAM demapping process to the data bursts.
A significant time delay occurs in this process, since the next stage can not be started until each stage is finished. It is shown in FIG. 3 that the delay of about 11 symbols occurs. This delay depends on the construction of the system. In particular, this delay increases along the length of the slot. Thus, the analysis for the frame prefix and the MAP information can not be started first, since the modulation is performed for each symbol before the channel decoder 27, however, the channel decoder performs decoding for each slot.
As a conventional method using an adaptive modulation and coding method, there is a Korea Patent Publication No. 2003-69299 entitled An adaptive modulation and coding method for a mobile communication system. The invention actively responds to the channel circumstance being changed in the wideband high speed mobile communication and adopts an AMC (Adaptive Modulation Coding) and an STC (Space Time Coding) at the same time.